Millennial-scale reworking of tephra in alluvial to shallow marine settings: Distinguishing pseudo-isochrons from genuine ones

Detection and Characterisation of Eemian Marine Tephra Layers within the Sapropel S5 Sediments of the Aegean and Levantine Seas

“…Of 16 glass shards analysed for geochemical composition from this layer ( Figure 5), one yielded a chemical signature similar to that of the overlying visible layer LC21 970.9 ( Figures 5 and 6), and may, therefore, have been derived from it by localised redistribution through bioturbation of shards. Nine of the shards revealed compositional data that closely resemble those of the underlying sample LC21 993.5 ( Figure 6), which might indicate reworking by the process described by [81]. The remaining 6 shards are geochemically distinct from all other samples in this study (Figures 5 and 6), and, therefore, may represent a primary tephra deposit.…”

“…However, some limited reworking by burrowing may have occurred as indicated by tephra sample LC21 959.5 which contains a mix of glass components from primary over-and underlying tephras. (3) Prolonged deposition of tephra in the vicinity of LC21 could have been fed by reworked material removed from the trough slopes that surround the core site, in a similar manner to the process proposed by [81] which appears to generate "pseudo-isochrons" in deltaic sediments on a millennial timescale (see [81], Figure 5). In this process the reworked tephra shards (and other sediment grains) can be co-deposited with primary pelagic sediment for a long time after the eruption event.…”

“…Explanation 3 is considered the most likely here, as the process of the redeposition of tephra shards into an otherwise intact foraminiferal isotope stratigraphy has been observed in other sedimentary records [81] whereas the processes invoked by Explanations 1 and 2 have not been recognised elsewhere. Explanation 2 is also contradicted by the evidence in [21,54].…”

The Volcanic-Rich Layer of the “Camporotondo (Marche, Italy)” Section: Petrography and Sedimentation of an Unknown Distal Messinian Eruption

“…Pyroclastic horizons can be either primary and secondary, if remobilized or altered after a first depositional process [1,6,[20][21][22][23][24]. Thereby, both fall and flow deposits after their original sedimentation can remain undisturbed (primary tephra), or be subject to gravity or flow remobilization or may be affected by chemical alteration processes by exogenous sedimentary mechanisms, i.e., secondary tephras [25][26][27][28][29][30][31][32][33][34][35]. Under subaerial conditions these deposits are easily eroded, whereas in submarine settings they are frequently buried and preserved [8,16,[36][37][38].…”

“…Secondary deposits may have greater thicknesses than their primary equivalents at the same distance from the vent [39]. All these characteristics are really and straightforward useful when a direct comparison of the deposit from proximal to distal areas is observable, otherwise it is not an easy task to understand if a tephra or volcanic rich layer is primary or not [20,27,37]. Furthermore, the remobilization process can leave traces such as: (i) changes in grain size distribution (GSD), (ii) change in grains shape, (iii) load-, flux and/or gravity-driven sedimentary structures, (iv) soft-sediment deformation and (v) presence of bioturbation and fossils [27][28][29][30]38,[41][42][43].…”